Sally has been a Holden's girlfriend for quite an extended period of time. She is cute, beautiful, attractive and sexy. Holden doesn't treat her with respect if comparing with the emotions he has to Jane. He is likely to have sexual interest instead of serious feelings to Sally Hayes.
She reads a lot, but Holden still calls her stupid. The readers can make their own conclusions about her level of intelligence when she rejects the idea to run away with Holden and live in the woods. Although Caulfield doesn't understand her point of view, she behaves more reasonably than her boyfriend.